1. Field of the Invention
The present invention relates to lightweight well cement compositions and methods, and more particularly, but not by way of limitation, to such compositions and methods for performing completion and remedial operations in wells.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in oil, gas and water well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe such as casing is cemented in a well bore. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior of a pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The cement composition utilized in primary cementing must often be lightweight to prevent excessive hydrostatic pressures from being exerted on formations penetrated by the well bore. Many lightweight cement slurries utilize water to reduce density; however, excessive water contributes to numerous undesirable slurry properties like solids settling, free water and low compressive strength. Solids settling within the cement slurry column results in nonuniformity of slurry density with accompanying free water. Lightweight additives like bentonite, sodium meta silicate, and the like control the free water, but do nothing to contribute to compressive strength development.
In carrying out completion and remedial cementing operations, the typical cement compositions utilized must have adequate pumping times before placement and short set times after placement. If a cement composition is slow to set, pressurized formation fluids can flow into and through the annulus before and after the cement composition sets. Such an occurrence is attributable to the inability of the cement composition to transmit hydrostatic pressure during the transition time of the cement composition, i.e., the time during which the cement composition changes from a true fluid to a hard set mass.
During the transition time of a cement composition, initial hydration of the cement composition has begun and the slurry starts to develop static gel strength. While the cement composition has little or no compressive strength, it becomes partially self-supporting which lowers the hydrostatic pressure exerted by the composition on pressurized fluid containing formations penetrated by the well bore. When the cement composition becomes partially self-supporting due to the development of gel strength prior to setting, volume reductions in the cement composition caused by hydration and fluid loss result in rapid decreases in the hydrostatic pressure exerted by the cement composition. The fluid phase within the cement matrix is not compressible and thus when the pressure exerted by the cement composition falls below the pressure of formation fluids, the formation fluids enter the annulus and flow through the cement composition forming undesirable flow passages which remain after the cement composition sets. If the formation fluids that flow through the cement composition include water which dilutes the composition, the ability of the composition to subsequently develop sufficient compressive strength and provide a competent seal can be reduced. The use of a highly compressible phase, like gas, in the cement composition improves the composition's ability to maintain pressure and thus prevent the flow of formation fluids into and/or through the composition.
The cementing problems mentioned above are aggravated in wells which are completed in deep water or in deep continental basins. In deep water or deep basins, casings must be cemented in well bores at locations sometimes many thousands of feet below the surface of the sea or land. Thus, the cement compositions utilized must remain fluid to allow adequate placement time without gaining substantial gel strength. After placement, the cement compositions must quickly develop gel strength and set, i.e., they must have short transition times in order to prevent pressurized formation fluids from entering the annulus or be capable of compensating for the cement compositions volume reduction by use of a compressible gas phase in the composition.
While cementing compositions and methods have been developed which have improved the success rate of well cementing operations, there is still a need for improved lightweight, fast setting, high compressive strength well cement compositions which are compressible and methods of using such compositions.